1. Field of the Invention
This invention relates to an antenna for use in, for example, a communication base station.
2. Description of the Related Art
Conventionally, the angle of beam orientation of a strip line is fixed.
This conventional feed line unitary antenna is exemplified by a microstrip antenna as shown in FIG. 29 of the accompanying drawings, which is reillustrated from "Handbook of Microstrip Antennas vol. 2" by J. R. James and P. S. Hall, pp. 1076, FIGS. 17 and 18, Peter Peregrinus Ltd., London, United Kingdom, 1989. In FIG. 29, reference numerals 1a, 1b designate a microstrip antenna; 2, an upper conductor of a microstrip line; 3, an earth conductor; and 4, a dielectric plate.
In FIG. 29, part of the electric power from the microstrip line 2 is supplied to the microstrip antenna 1a, and then the electric power passing through the microstrip antenna 1a is supplied to the next microstrip antenna 1b. As the individual microstrip antennas 1a, 1b are excited by certain amplitude and phase distribution, the antenna apparatus forms a beam pattern in space. However, the known antenna apparatus has a problem that the angle of beam orientation cannot be varied in the same frequency, as long as the shape of the antenna system such as the length of the feed line and/or the spacing of the antennas are changed. In general, in order to scan the antenna beam, each antenna element is equipped with a phase shifter; however, no low-cost small-size phase shifter suitable for the antenna of FIG. 29 is known at the present time.
FIG. 30 shows an antenna system for performing communication between a number of mobile stations and a data terminal or telephone using the antenna apparatus of FIG. 29.
In FIG. 30, reference numerals 100-102 designate mobile stations each equipped with a transmitter/receiver for communication with another station using a different frequency. A base station (fixed station) 103 includes, transmission/receiving antennas 1, 1', a local oscillator 105, a transmission modulator 104 for modulating transmission signals by a high frequency signal of the local oscillator 105, and first and second receiving demodulators 106, 107 for demodulating receiving signals by a high frequency signal of the local oscillator 105. The base station 103 further includes a line connector 108, a controller 109 for controlling the line connection of the line connector 108, and a communication processor 110 for processing transmission data of the mobile station 100-102 and other data from a data terminal 112, another base station 113, a telephone, etc. The switching between transmission and receiving modes of the base station 103 is performed by switches S1, S2. For receiving transmission data of the mobile station 100-102, as shown in FIG. 30, the antennas 1, 1' are connected with the receiving demodulators 106, 107. For sending predetermined data to the mobile station, the individual switches S1, S2 are connected with the sending modulator 104. During transmission, a switch S3 selectively connects the transmission modulator 104 with one antenna 1 or 1' via the switches S1, S2. The switch S3 selects one of the antennas 1, 1' according to the output level of the first and second receiving demodulators 106, 107, and this switching is controlled by the controller 109. In the antenna system, data received from the mobile station 100-102 is transmitted to the communication processor 110 via the antennas 1, 1', the switches S1, S2, the first and second receiving demodulators 106, 107 and the line connector 108, whereupon the data processed by the communication processor 110 is transmitted to the terminal 112, another base station 113 or the telephone 114 via the public communication network 111. Meanwhile, the data from the terminal, another base station 113 or the telephone 114 is transmitted to the mobile station 100-102 from one of the antennas 1, 1' via the public communication network 111, the communication processor 110, the line connector 108, the transmission modulator 104, the switch S3 and the switch S1 or S2.
In recent years, application of feed line unitary antennas has been on the increase in order to minimize the size of the antennas in the base station. However, with this kind of antenna, the angle of antenna orientation is decided only by the length of the feed line or the spacing of the antenna elements so that for varying or controlling the angle of the antenna, it has been conventional practice to mechanically rotate the antenna.